Hybrid fiber coaxial (HFC) cable television systems have been in widespread use for many years and extensive networks have been developed. A typical HFC network generally contains a headend which provide communications between user in the HFC network and the IP/PSTN networks. The headend usually contains a cable modem termination system (CMTS) containing several receivers, each receiver handling communications between hundreds of end user network elements. The headend is generally connected to several nodes and each node is connected to many network elements, such as data over cable system (DOCSIS) terminal network elements (e.g. media terminal adapters (MTA) or cable modems), e.g., a single node may be connected to hundreds of modems. In many instances several nodes may serve a particular area of a town or city.
A typical HFC network uses optical fiber for communications between the headend and the nodes, and uses coaxial cable for communications between the nodes and the end users. Downstream optical communications over the optical fiber are typically converted at the nodes to RF communications for transmission over the coaxial cable. Conversely, upstream (or return path) RF communications from the users are provided over the coaxial cables and are typically converted at the nodes to optical communications for transmission over the optical fiber. The return path optical link (the optical components in the HFC network, e.g. the transmission lasers, optical receivers, and optical fibers) contribute to the performance of the HFC network. More particularly, the optical components contribute to the quality of signals received by the CMTS from the users, and may cause distortion of the signals or otherwise degrade their quality.
The RF signals are generally in the form of modulated RF signals. Several modulation schemes exist with different levels of complexity. However, phase noise or narrow band interference may prevent a HFC network from effectively using some of the modulation schemes. Particularly, phase noise and/or narrowband interference will often limit the level of modulation complexity that may be transmitted over an upstream HFC plant. Phase noise is generated through the modulation and demodulation processes between the DOCSIS terminal devices (MTA or cable modems) and the CMTS. This noise generally combines with and potentially degrades the noise already present within the upstream HFC plant. Distortion is typically generated by nonlinear active components in the return path. Distortion may produce appreciable narrowband spurious signals, which could interfere with frequencies used by the CMTS and DOCSIS terminal devices. Accurately diagnosing phase noise and/or narrowband interference issues requires a technician or engineer to be at multiple locations within the HFC plant simultaneously to inject test signals at the suspected DOCSIS terminal device locations and assess performance at the headend location with specialized test equipment, such as a vector signal analyzer. The problem with this diagnostic process is that it is manual, time consuming and costly.